The present invention relates to an intra-vascular imaging method and apparatus which allow the acquisition of endoscopic images of small internal cavities of the body including methods and apparatus for visualization through opaque liquid media.
Heart and blood vessel diseases are among the main causes for morbidity and mortality in Western society. Therefore, interventional procedures involving blood vessels of the heart are among the most widely used in the medical field. The pathology that is in the base of most acute coronary syndromes and sudden cardiac deaths is atherosclerosis. In this process, atherosclerotic plaques, which are an active collection of different cells, mainly immune cells and smooth muscle cells along with deposits of fatty substances, cholesterol, cellular waste products, calcium and other substances, are accumulated in the inner lining of an artery. Stable plaques, which cause the more significant narrowing of the arterial wall, are considered the major factor in the development of angina pectoris (chest pain). However, studies from recent years have shown, that unstable angina, myocardial infarctions (heart attacks) and sudden cardiac related deaths are caused mainly by unstable plaques, otherwise known as vulnerable plaques. This type of plaque is usually smaller and therefore less significant and difficult to detect with currently used angiographic methods described hereafter.
Some of the important developments were made in the field of minimally invasive procedures. A very common diagnostic and therapeutic procedure is cardiac catheterization. The commonly applied method, angiography, includes imaging the heart and coronary blood vessels using an X-ray camera as the imaging device, and a catheter, through which a contrast substance is injected into the heart and vessels to enable them to be viewed by the camera. This method gives a two-dimensional monochromatic view of the heart and blood vessels as viewed from the outside. This method detects major occlusions by identifying places where blood flow is disturbed and it may direct the PTCA (Percutaneous Transluminal Coronary Angioplasty) or stent-inserting technique to the place of the occlusion, but it does not give a direct view of the occlusion site or the surrounding area. One of the major risks of the techniques described above is a rupture or a disruption in the fibrous cap covering the plaque and the release of plaque particles into the blood stream. These particles may cause numerous small occlusions in the coronary arteries but also may cause occlusions in small blood vessels of other organs, such as the brain, kidney, or lungs. A direct, clear view of the field of operation, as provided in the current invention, could substantially decrease the risk of disruption, as described above. Also, and perhaps more importantly, only through intra-vascular imaging will it be possible to detect the smaller, vulnerable plaques. The effectiveness and precision of the plaque treatment, when assisted with direct intra-vascular imaging, such as in the present invention, would be enhanced when compared to current indirect imaging methods.
Important methods that have been developed to confront the issue of intra-vascular imaging are angioscopy and intra-luminal ultrasound. New techniques, which are still under development, include Optical Coherence Tomography (OCT) and infrared endoscopy.
Angioscopy is a form of endoscopy developed for the arteries. Because the illumination used in angioscopy is in the visible wavelength range, in which the blood that fills the arteries is opaque, the method requires a way of moving the blood from the field of view prior to visualization. One way to do this is by injecting a high-pressure physiological fluid into the vessel to temporarily displace the blood, as disclosed in U.S. Pat. No. 4,827,907, U.S. Pat. No. 4,998,972, U.S. Pat. No. 5,730,731, U.S. Pat. No. 5,010,875 and U.S. Pat. No. 4,934,339. Another way of clearing the field of view is by inflating a balloon, which is positioned at the distal end of the angioscope, in front of the camera-head or optical assembly. The balloon is made of a transparent substance, so that when it is inflated inside the blood vessel, with either gas or a transparent liquid, it pushes the blood away from the distal end of the angioscope and clears a field of view of the walls of the vessel. Such an apparatus is described in U.S. Pat. No. 4,784,133 and U.S. Pat. No. 5,411,016; the latter patent disclosing a transparent part at the distal end of the angioscope in addition to the balloon surrounding it. A similar apparatus is disclosed in U.S. Pat. No. 4,470,407, except that the optical system terminates inside the balloon (also allowing laser operation through the balloon). An apparatus that uses two spaced and expendable balloons, that occlude and isolate an operating area in the blood vessel between them, is disclosed in U.S. Pat. No. 4,445,892. Most methods combine an inflatable balloon with injection of a transparent liquid. The balloon coaxially surrounds the sheath at the distal end of the catheter and, when inflated, it blocks some of the blood flow. The method described above allows the injection of less flush liquid and at a lower pressure, which is safer and more efficient. Prior art in which the method described above is used in U.S. Pat. No. 4,576,145, U.S. Pat. No. 4,576,146, U.S. Pat. No. 5,263,928 and U.S. Pat. No. 5,464,394. A combination of an angioplasty balloon with intra-vascular endoscopy is disclosed in patents EP177124A, U.S. Pat. No. 5,116,317 and U.S. Pat. No. 4,961,738. In the latter patent, the optical system terminates within the balloon and there is a “working well” in the balloon to allow the insertion of instruments into the lumen of the vessel.
Another method for intra-vascular imaging is the use of ultrasound. The ultrasound transducer is positioned at the distal end of a catheter inside the blood vessel and the ultrasound transducer is used to obtain an image of the lumen and walls of the artery. Patents referring to this kind of apparatus are U.S. Pat. No. 6,129,672, U.S. Pat. No. 6,099,475, U.S. Pat. No. 6,039,693, U.S. Pat. No. 6,059,731, U.S. Pat. No. 5,022,399, U.S. Pat. No. 4,587,972, U.S. Pat. No. 4,794,931, U.S. Pat. No. 4,917,097 and U.S. Pat. No. 5,486,170. A patent that combines PTCA with ultrasonic imaging is U.S. Pat. No. 5,167,233.
OCT provides a three-dimensional image by performing optical measurements, and it can be used in intra-vascular imaging. Related patents are U.S. Pat. No. 6,134,003, U.S. Pat. No. 6,010,449, and U.S. Pat. No. 5,459,570.
The opaqueness of blood at visible light wavelengths poses a specific problem when attempting to acquire an image of an intra-vascular space. One solution to the problem noted above is to utilize infrared (IR) light to enable visibility through the suspended particles and cells in the blood. A patent that discloses a method for using deep-IR light for imaging through blood is U.S. Pat. No. 6,178,346. The use of deep-IR wavelengths to achieve visibility in a blood medium as described in the referred patent requires very high-energy illumination, which has risks and disadvantages when used inside the body. The use of near-IR radiation substantially diminishes risks. U.S. Pat. No. 4,953,539 discusses the use of an endoscopic imaging device, which is illuminated from outside the body with infrared light. The referred patent serves as an example of the use of infrared light in imaging body organs. External illumination has not been used to date for intra-vascular imaging.
A well-known property of human tissue is that it has different absorption, scattering, and attenuation coefficients of IR radiation. This fact allows different types of tissues to be distinguished in general, and allows different types of plaque to be to be distinguished in particular. Reference is made to “A Review of the Optical Properties of Biological Tissues” Cheong, Prahl and Welch, IEEE J. of Quantum Electronics, Vol 26 No 12 December 1990.